Bridge arrangement



octzs, 1940. T A -RICH v.2,219,939r

BRIDGE ARRANGEMENT Filed Marh l, 1939 Tl'mecndoheAizlw,

' HIS Attorney.

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Patented Oct. 29, 1940 UNITED STATES `Baroda ARRANGEMEN'I'v Theodore A. Rich, Schenectady, N.' Y., assignor to General Electric Company, a corporation of New York y Application March 1,

2 Claims.

My invention relates to electrical measuring instruments and concerns particularly apparatus for measuring impedance and resistance.

It is an object of myinvention to provide a 5 self-balancing bridge for measuringresistance or for measuring impedance in which the indicating element draws no current in the positionv of balance and in which the apparatus is independent of variations in voltage or in frequency.

It is also an object of my invention to provide such apparatus for measuring temperature.

vAnother object of my invention is to provide apparatus. responsive to variations in resistance, impedance or temperature and which may be energized by alternating current circuits.

Other and further objects and advantages will become apparent as the descriptionproceeds.

In' carrying out my invention in its preferred form I arrange a Wheatstone bridge circuit with the stationary coil of a dynamometer instrument serving as one of the arms of the bridge and the .resistance or impedance to be measured as another arm of the bridge and I connect the movable coil of the instrument as the cross arm of the bridge so that the movable coil has impressed across it the voltage due to bridge unbalance, at the same time there may be a voltage induced in the movable coil due to current owing in the stationary coil. The arrangement is such that deflect 'the movable coil in opposite directions and to bring the coil to a position of rest in which the voltages are equal and opposite so that there is no torque producing current in the moving coil. vThe indication of the instrument, accordingly, represents the degree of unbalance of the bridge circuit, which depends on the resistance measured, and since no current is fiowing, the indication is independent of voltage and frequency of the supply source. n A

The invention maybe understood more readily from the following detailed description when', considered in connection with the accompanying drawing and the features of the invention which are believed to be novel and patentable will be pointed out in the claims appended hereto.

In the drawing Fig. l is a circuit diagram schematically representing `one embodiment of my invention and Fig. 2 is a circuit diagram of another embodiment of my invention which is particularly adapted to a. measurement of'low resistance. Figs. 3 and 4 are circuit diagramsof other embodiments of my invention designedto operate upon low frequency circuits without sacthe currents produced by the two causes tend to 1939, serial No. 259,206 (ci. r11-95) rice of sturdiness and ruggedness of construction.. f

"In the arrangement of. Fig. 1 I have represented at II a current carrying element the resistance of which is to be measured. The element Il is connected in series with a standard resistor I2 of xed resistance to form two resistance arms of a bridge. A source ofl alternating 4current I3 is connected thereto for energizing the bridge. A dynamometer type instrumentA I4 Whichmay be of the type described in VUnited States Patent 1,976,498--Hoare and Parker, for example, is provided having a stationary coil or' 'winding I5 on a laminated iron yoke I5' and a movable 'coil or winding I6. A reactor Xor. ini ductor I1.is also provided which is connectedin series with the winding I5 to the current source I3'to form inductive arms of a bridge. The movable coil IS is connected between the common point I8` o f the vbridge arms II and I2 and the common point I9 of the bridge arms I5 and I1 to form a diagonal or cross arm of the bridge. The' dynamometer linstrument I4 may be conf dynamometer type devices known to those skilled in the art having, for example, a shaft, not shown, upon which the'movable coilI I6 is mounted so as to be`rotatable and carrying a pointer 20 cooperating with a scale 2|. My` invention, however, is not limited to use with aniindicating instrument and obviously includes use with any 4 suitable current responsive device such as recrdera-Ytelemeters and contact-making instruments. It will be understood -that' aL suitable means is required for conducting they current to the coil I 6, such as lead-in spirals, not shown, but for maximum sensitivity in my apparatus the movable element I 6 is preferably arranged without biasing springs and the lead-in springs are preferably of negligible torque so that the movable element `is substantially free to oat in any position. On the other hand, alight biasing spring arranged to sweep the pointer to or beyond one end of the scale or the other in case of voltage failure may be provided if it is desired to prevent the moving element from remaining in the position which it last reached, for in the case of voltage failure changing the value of the impedance' to be measured would have no effect on the instrument indication.

The principle of operation ofthe apparatus will be apparent from the following considerations. Since the bridge arms I5 and' l1 are both inductive, the voltages therein will both` lead the current `bv snbstantinllv Qn damning structed' in the manner of current responsive Y ductance.

`through either. by any voltage difference between the points I8 and will, therefore, have substantially `the same phase relationships so that they will be substantially in phase with the voltage of the source I3'. Since'the arms I I an I2 are substantially pure resistance the voltages herein will. also be substantially in phase with the voltage of the source I3. Consequently the voltage between the points I8 and I9 will .be either in phase or substantially 180 degrees out of phase with the voltage of the supply source in case the bridge should be unbalanced.'

Current owing in the stationary coil I5 of the dynamometer instrument" I4 tends to induce a current in the movable coil I6 except when the movable coil I6 is in the position illustrated,

which is the position of minimum mutual in- If the coil I6 is not in the position of minimum mutual inductance, the direction of the current induced is such as to deflect the coil to the position of minimum mutual inductance in accordance with the well-known electromagnetic principle that two relatively movable inductive coils tend to assume the position of minimum mutual inductance when current is passed Current produced in the coil I6 and I9, however, tends to deflection of the coil I6 from the position of minimuml mutual inductance to the position of maximum inductance by motor action in accordance with the principle of operation of' a watt meter, and such current tends to flow inthe opposite direction to the current induced by the current in the stationary coil I5. Accordingly, the coil I6 is deflected from theposition of minimum mutual inductance to the position inv which'suiiicient current is induced by the stationary coil I5 to balance exactly the current produced by voltage difference' between the points I8 and I9 and the position `assumed represents the degree of.unbalance of the bridge serving as a measure of the resistance I I. .Since in this position no net current is flowing in the coil I6, it will b e apparent its position is not dependent upon the magnitude of the current supplied by the source I3. IThe voltage ratios are not affected by variations in voltagel fore independent of variations in voltage and fre-` quency of the supply source.

It has been assumed in the foregoing explanation that the arms II and I2 are pure resistances and that the arms I5 and I1 are pure-inductances. However, it .will be understood that the same explanation would be applicable if the arms II and I2 were'impedances with either inductive or capacitive components having substantially the same power factor, and if the arms I5 and I'I had some unavoidable resistance, but substantially the -same power factor. Although I prefer to have the series connected ,arms with power factor substantiallyequal in order to obtain the maximum sensitivity for .a given limiting current and in order to obtain the most accurate calibration of the instrument, it will be understood that exact quality is not necessary since lack of uniformity in power factor in either pair of series bridge arms will result primarily in out-of-phase current components in the cross arm paratus `willl still operate to indicate, as distingu'ished from Vaccurately measuring, unbalance of the circuit even with widely different power factor of the various bridge arms.

When the apparatus is to be used for measurement of temperature, the resistance arm Il is preferably composed of material which varies in resistance with temperature and is placed 4at a position where the temperature is to be measured.

Although the apparatusl is independent of theV effect of variations in voltage and frequency, I

find it easier to construct rugged, sturdy instru-4 ments where the frequencies exceed` the usual commercial frequencies. vFor example, at a frequency of approximately 800 cycles rugged, inexpensive instruments may readily be constructed. However, the apparatus may also be employed with either considerably higher or considerably lower frequencies. If it is desired to obtain the advantages of maximum economy of construction of the -in'strument when low frequency circuits are to be used for energizing the'apparatus, I may interpose frequency multi- `plying devices between the input to the bridge which have Ano effect. Furthermore, the ap and the current source. For example, in the arrangement illustrated in Fig. 3, a frequency multiplying transformer 22 which may, for example, beof the saturated secondary core or peaking type is provided, which is connected on the primary side to the alternating current source I3 and on the .secondary side to the input terminals 23 vand 24 of the bridge 25. In connection with three-phase circuit the well-known frequency-triphasing transformer may be used.-

Instead of utilizing the frequency multiplying transformer 22 I may interpose a device 26 in series with source I3 of the bridge 25 for introducing high frequency harmonics or components in the current supplied to the bridge Y25. devicef26 may'take the `formiofa saturated reactor or of a non-linear resistance such as that describedv in the Patent 1,822,742 to McEachron in which the ratio of rvoltage to'current varies with voltage so that numerous odd harmonic components may be introduced as explained in the copending application ofI Cramer W. La Pierre Serial Number 65,998, filed February 2'7, 1936, and assigned to the same assignee as the present application. The resistance material employed may be a mixture of silicon carbide 'and carbon with a suitable binder or silicon carbide mixed with other conducting materials suchas tungsten, molybdenum, and the like. A

frequency multiplication.

If it is desired to utilize a high resistance bridge or a high voltage circuit for measuring low values of impedance or resistance, the arrangement of Figure 2 maybe employed in which one of the arms I I' of the bridge consists of a high voltage winding 21 of a transformer having a low voltage winding 28 to which an impedance 29 to be measured is connected having a low ohmic value.

In order to reduce the volt-ampere burden on the current source I3 a condenser 30 may, if desired, be connected in series with the source I3 and the bridge terminals 23 and 24.

In accordance with the provisions of tlie patent statutes I have described the principle of operation of my invention, together with the apparatus which I now consider to represent the Thel best embodiment thereof, but I desire to have it understood that the apparatus shown is only il.- lu'strative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. Self-balancing apparatus of the bridge type comprising in combination, a source of a1ter nating current, a frequency multiplying transformer of the saturated secondary core type having a primary winding connected to the 'source of alternating current, and having a secondary winding, a pair of impedances connected to thc secondary winding in seriesv to form two outside arms of a Wheatstone bridge, a dynamometer type indicating instrument having a pair of relatively movable coils, an impedance connected in series with one of said relatively movable coils to said transformer secondary winding to form the two remaining outside arms of the `Wheat stone bridge, the remaining coil of said instrument .being connected between the common points of the series connected elements to form a cross arm of the bridge, whereby currents owing in the arms of the bridge including the instrument coil induce current inthe cross-arm coil of the instrument, tending to bring the mstrument to the position of minimum mutual inductance, and any currents in the cross arm produced by unbalance of the bridge tend to bring the coils in the position of maximum mutual inductance so that the instrument assumes the position in which the currents are balanced,

thereby indicating bridge balance independently of voltage or frequency fluctuations of the supply source.

2. Self-balancing apparatus of the bridge type comprising in combination, a source of alternating current, a pair of impedances operatively connected in series relation thereto -to form two opposite arms of a Wheatstone bridge, means ininstrument beingl connected 'between the common points of the series connected elements to form a cross arm of the bridge, whereby currents flowing in the second pair of arms of thebridge induce current in the cross-arm coil of the in,- strument, tending to bring the instrument tothe position of minimum mutual inductance, and any currents in the cross arm produced by unbalance of the bridge tend to bring the coils in the position of maximum mutual inductance so that the instrument vassumes the position in which the currents are balanced, thereby indicating bridge balance independently of voltage or frequency 30 fluctuations of the supply source.

THEODORE A. RICH. 

